<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>菜鸟教程(runoob.com)</title>
    <script>
        var c = 0;
        var t;
        var timer_is_on = 0;
        function timedCount() {
            document.getElementById('txt').value = c;
            c = c + 1;
            t = setTimeout("timedCount()", 1000);
        }
        function doTimer() {
            if (!timer_is_on) {
                timer_is_on = 1;
                timedCount();
            }
        }
    </script>
</head>

<body>
    <form>
        <input type="button" value="开始计数!" onClick="doTimer()">
        <input type="text" id="txt">
    </form>
    <p>单击按钮，输入框将从0开始一直计数。</p>
</body>

</html>